The present invention relates to devices utilized in optical transmission systems. In particular, it relates to polarization encoding of optical signals as a function of wavelength.
With the development of technologies such as wavelength division multiplexed (WDM) optical communication systems, a need has been created for inexpensive, efficient means to effect wavelength dependent processing of optical signals, for example so that individual channels may be manipulated. Examples of such processing include wavelength selectable optical switching, useful in routing individual wavelength channels for WDM systems, and wavelength dependent power attenuation, which might be used for gain equalization in a WDM system.
One known approach to wavelength dependent processing of optical signals is to first separate, or demultiplex, the individual channels and then to process each channel separately. The processed channels may then be multiplexed back into a single fiber, separately distributed, or otherwise manipulated.
A disadvantage to demultiplexer and multiplexer based devices is that insertion losses are generally high. Also, devices based on demultiplexer geometry tend to be bulky and expensive. Therefore a new approach is needed in which no multiplexer or demultiplexer operation is needed.
The inventors have recognized uses for a device that can alter polarization of an optical signal, and in particular that can alter polarization of the optical signal differently for different wavelength components of the signal. Such a device could be used in applications including distortion correction, optical power level adjustment and tunable optical switches that are capable of avoiding multiplexing/demultiplexing steps.
Birefringent materials are known to be able to impose wavelength dependent polarization characteristics.
Single waveplates, however have limited application. A fixed stacked group of birefringent elements is able to allow manipulation of a particular group of channels, but does not allow arbitrary alteration of the output as for example discussed in Harris, Amman and Chang, xe2x80x9cOptical Network Synthesis using Birefringent Crystals. I. Synthesis of Lossless Networks of Equal-Length Crystals,xe2x80x9d and Amman and Chang, xe2x80x9cOptical Network Synthesis using Birefringent Crystals. II. Synthesis of Networks Containing One Crystal, Optical Compensator, and Polarizer per Stage.xe2x80x9d
Such a fixed stack of birefringent elements have been used in conjunction with a digital polarization converter to allow some control over the transfer function of the device. In particular, stacked birefringent elements have been tried as a solution to all-optical switching based on polarization as shown, for example, in U.S. Pat. No. 5,694,233 to Wu, et al. (xe2x80x9cthe ""233 patentxe2x80x9d).
The inventors have therefore identified a need for a device to selectively and controllably change a polarization state of selected ones of a plurality of channels, thereby allowing various further processing of a an optical signal, among other useful results.
An optical polarization encoding device has been invented which allows wavelength dependent processing of polychromatic optical signals without prior separation into narrow wavelength bands.
One embodiment of the invention includes a polarization independent polarization encoder including a first birefringent component configured and arranged to accept an input beam of light having a plurality of channels included therein and to divide the input beam of light into a first intermediate beam and a second intermediate beam, the first and second intermediate beams having respectively orthogonal polarizations, a second birefringent component configured and arranged to controllably impart a respective wavelength dependent polarization to each of selected ones of the channels in each of the intermediate beams, and a third birefringent component configured and arranged to combine the first and second intermediate beams into an output beam having the plurality of channelsat least one wavelength channel therein, each of the selected ones of the channels having a respective wavelength dependent polarization.
Another embodiment of the present invention includes a polarization encoder including a birefringent component configured and arranged to accept an input beam of light having a plurality of channels therein and to controllably impart a respective wavelength dependent polarization to each of selected ones of the channels, said birefringent component comprising a plurality of controllable variable birefringent elements.
Another aspect of the present invention includes a method of polarization encoding of an input beam of light having a plurality of channels included therein, including receiving an input beam of light, controllably imparting a polarization state to selected ones of the channels, and transmitting an output beam of light containing the polarized selected ones of the channels.
Yet another aspect of the present invention includes an optical transmission system including an input optical waveguide configured to accept an input optical signal having a plurality of channels therein, a polarization encoder in optical communication with the optical waveguide, the polarization encoder including a birefringent component configured and arranged to accept an input beam of light having a plurality of channels therein and to controllably impart a respective wavelength dependent polarization to each of selected ones of the channels, said birefringent component comprising a plurality of controllable variable birefringent elements and an output optical waveguide configured to accept an output optical signal from the polarization encoder.